In an orthogonal frequency division multiplexing (OFDM) based communication method, interference does not exist between different users since the different users use different frequencies for data channel. A method for using the different frequencies for the different users is referred to as frequency division multiplexing (FDM). However, code division multiplexing (CDM) that identifies the users by using codes is appropriate for a reference signal or a control channel. The CDM is superior to the FDM since the CDM can efficiently use resources for the reference signal or the control channel. The CDM is classified into a time domain CDM that directly spreads by OFDM symbol units and a frequency domain CDM that spreads in a frequency domain within one OFDM symbol.
In the frequency domain CDM, the different users multiply the same sequence by complex sine waves having different phase slopes before transmitting the same sequence. At this time, a reason why the complex sine wave is used instead of a Hadamard matrix is because orthogonality between the users can be guaranteed through a signal processing at the receiver. Since multiplying any sequence by a complex sine wave with a phase that linearly increases with frequency at the frequency domain is the same as cyclic-shifting in the time domain, a process for multiplying the complex sine wave is referred to as a cyclic-shift process or a cyclic delay process.
The number of users that can be simultaneously admitted in, one OFDM symbol when the frequency domain CDM is used may be given by a function of a frequency difference between subcarriers and delay spread in the propagation channel. At this time, granularity of the cyclic-shift of each user is set to be greater than a delay spread in the propagation channel. In a cellular communication environment, the value of the maximum delay spread is arbitrarily set since the maximum delay spread is difficult to be estimated.
Accordingly, a channel delay spread of any user may be greater than the cyclic-shift in the cellular communication environment and so, the interference between users may occur.